First of all, don't get rid of the dog. Find a trianer to work with the dog. What kind of breed is your sister's dog? How old is he? One more question, was he trained to fight? If he was then he will be a nasty dog. But please don't get rid of him. Find a trainer and work with him. Underneath, he is a wonderful dog. You just need to find that. Give him lots of love. Hope this helps. Good luck!